A toroidal volume search sonar (TVSS) has successfully been used in detecting mine-like objects in shallow water. The toroidal shaped array configuration used in the TVSS has advantages over a standard linear shaped array for mine detection in shallow water because the TVSS configuration allows the construction of very narrow (vertical width) beams, which can penetrate deeply into a shallow water column. Exemplary of the prior art TVSS is the description in the U.S. Pat. No. 5,506,812 to W. J. Zehner. A beamforming methodology for TVSS in shallow water is disclosed in U.S. patent application Ser. No. 08/869,722, now U.S. Pat. No. 5,781,504, filed Jun. 5, 1997, and assigned to the same assignee as the present application.
For each sonar ping, the beamformed output of a TVSS consists of many radial receive beams. The large number of beams present the sonar operator with a considerable amount of visual information which must be assessed at every ping (typically once a second). The standard polar display for each ping of a TVSS displays the range and radial beam angle to a target to provide information about where in the water column a target is located. However, some form of a time (ping) history would greatly aid a sonar operator in identifying a target of interest since anomalous events could more easily be distinguished from a background reference over a time period covering many pings. Yet, because of the two-dimensional radial nature of the TVSS display, a time history of past radial images is difficult for an operator to view since it would involve a three-dimensional image (range, beam angle and time). Thus, there is a need for some form of data compression if TVSS data is to be presented as a function of time on a two-dimensional display.